The present invention relates to a wound electrode assembly for an electrochemical cell, and more particularly, to a wound electrode assembly wherein a portion of an exterior face of a substrate or grid of that portion of an electrode plate defining the outermost or peripheral layer or wrap of the electrode assembly is substantially exposed and contacts at least a portion of a container as assembled in a sealed electrochemical cell.
Conventionally, wound electrode assemblies for electrochemical cells are formed from two separate electrode plates of opposite polarity with a layer of interposed separator material. The negative electrode plate can be of a pressed or pasted design. An aqueous mixture of an electrochemically active material and a binder may be applied to each face of an electrically conductive, perforated substrate and pressed on to the substrate, for example by passing the substrate between rollers. The substrate can be stippled to improve adhesion between the substrate and the electrochemically active material Alternatively, both the positive and negative electrodes may be of a sintered design. A perforated or wire mesh steel substrate of, for example, 2-3 mils thickness, is sintered with a carbonyl nickel powder layer or layers to form a porous electrode plate of, for example, 20-30 mils thickness. The resultant porous plate is conventionally impregnated with a solution of an electrochemically active material. The electrochemically active material is precipitated out of solution within the plate.
In addition, an ultra high porosity positive electrode plate can be utilized in which an electrochemically active material may be bonded to a porous substrate such as a highly porous metal foam by, for example, pressing a slurry or paste containing the active material on to and within interstices of the substrate. The substrate may then be compacted to form a positive electrode plate having a desired thickness.
In one method of manufacturing cylindrical cells, a negative electrode plate, a positive electrode plate, and two pieces of separator are wound into a spiral such that surfaces of the positive and the negative electrode plates are juxtaposed throughout the cell. These cell components are wound around a removable arbor which is positioned within a nest having a diameter approximating that of the container of the electrochemical cell. As thus conventionally manufactured, the outer wrap of the electrode assembly is a layer of separator material which prevents the electrochemically active material on the outer periphery of the electrode assembly from being scraped off as the assembly is stuffed into a container during cell manufacturing.
Electrical contact between the electrode plates and the cell container of a wound electrochemical cell has been made by connecting one of the electrode plates to the sidewall or bottom of the cell container or housing while the electrode plate of opposite polarity is electrically connected to the top of the container or housing which is electrically insulated from the bottom or sidewall thereof. When an electrode plate is electrically connected to the bottom of a cell, a current collector tab which may be integrally formed in or which is attached, for example, by welds, to the electrode plate is secured to the cell bottom, for example, by inserting a welding tip through the center of the electrode assembly to weld the tab to the bottom. Such assembly is relatively difficult and slow. Alternatively, the collector tab can be bent back upon the wound electrode assembly to make pressure contact with the container or housing as disclosed in U.S. Pat. No. 4,049,882.
The electrode plate which forms the outer wrap or periphery of the electrode assembly may be electrically connected to the sidewall of the cell container or housing by contiguity. Such contact may be made by removing at least a portion of the outer layer or wrap of separator material thereby exposing the outer wrap of the electrode plate for contact with the container sidewalls. Thus, the outermost layer of electrochemically active material of the exposed electrode plate contacts the container sidewall and provides electrical communication between the substrate of the outer wrap of the electrode plate and the sidewall. U.S. Pat. No. 4,663,247 discloses a sealed galvanic cell wherein the outer exposed electrode of a coiled electrode assembly makes electrical contact with the wall of the container within which the assembly is disposed through an opening in the outer separator layer. U.S. Pat. No. 4,259,416 relates to a spiral electrode which is housed in a battery vessel and has the positive plate located at the outermost circumference of the spiral electrode electrically connected to the vessel by means of contact pressure. However, the electrochemically active material present on the electrode plate defining the outermost wrap or layer of conventional wound electrode assemblies possesses inherent electrical resistance which results in cell inefficiencies. In addition, such active material does not enter into electrochemical reactions occurring within the cell between opposing faces of positive and negative electrode plates during charge and discharge cycles.
Accordingly, it is an object of the present invention to provide a wound electrode assembly for use in a sealed electrochemical cell which provides for direct electrical contact between a portion of the substrate or grid of one electrode plate thereof and a portion of the cell container.
Another object of the present invention is to provide a wound electrode assembly which can be easily manufactured and as assembled within a sealed electrochemical cell will result in a substantially higher coulombic cell capacity and improved cell performance characteristics.
A further object of the present invention is to provide a wound electrode assembly which is easily assembled within a sealed electrochemical cell and, as assembled, simplifies the construction of the electrochemical cell.